What was Abraham Lincoln's first main idea? |

This is a rather vague question, but I will try to guess at what you mean.

I think that you might be asking what his main goal was in the things that he did as president.  If so, I would say that his main goal was not to end slavery, as many people think, but rather to keep the Union together.  He wanted to prevent the US from splitting up.

Lincoln himself said that his main goal was to keep the country unified.  He once said that if he could do it by freeing the slaves, he would, but that he would also do it by not freeing the slaves, if that is what it took.

Lincoln's single most important goal was to keep the US a unified country.